1. Field of the Invention
This invention relates to a mold used for the manufacturing of optical glass parts such as a lens and prism by directly pressing lumps of optical glass materials to form such optical glass parts.
2. Description of the Prior Art
In recent years, there is a tendency for the optical glass parts, especially the optical glass lenses, to have aspherical surfaces so as to simultaneously achieve the simplified construction as well as the lightweight lens of a lens system in an optical instrument. With the existing grinding method employed as a manufacturing method of the optical lens, difficulties are involved in the workability and quantity production while the direct press molding method is regarded as a promising method for manufacturing an aspherical lens.
In this direct press molding method, a pair of molds 1 and 2, e.g., as shown in FIG. 1, which is finished beforehand to the desired surface quality and surface accuracy and has aspherical pressing surfaces 3 and 4 as shown in FIG. 1 is used for molding by heating the lump of the optical glass on the mold or by pressing the lump of the preheated optical glass, without requiring any further post processes such as a grinding process after the press molding of the optical lens.
However, said manufacturing method of the optical glass lens is required to provide the produced lens with good quality to an extent that the quality of the image formed by the lens is not damaged after the press molding.
Therefore, the mold is required to have the least chemical action to the glass at high temperature, hard glass pressing surface which is not susceptible to damage such as scratches, and high breaking resistance to the thermal shock caused by repeated rapid heating and rapid cooling, among other properties.
To meet such requirements, materials such as silicon carbide and silicon nitride are said so far to be suitable for the mold, and various studies are being made.
However, many polymorphisms are present in the crystal structure of silicon carbide which is regarded as the most prospective material for the mold used for the above purposes and such polymorphisms extremely differ in their chemical and physical properties. There is a problem among others that some of such polymorphisms have violent reactions against the components of glass composition, and it cannot be simply said that silicon carbide is a suitable material for the mold by merely verifying that it is a compound of Si and C through analysis.